Electron emissive materials



Agg. 4, 1931. F. MEYER E'r AL 1,817,636

ELECTRON EMMISSIVE MATERIALS Filed June 6, 1927 Patented Aug. 4, 1931 UNITED STATES PATENT oFFlcE FRIEDRICH MEYER AND HANS-JOACHIM SPANNER, F BERLIN-HALENSEE, GERMANY, ASSIGNORSA, BY MESNE ASSIGNMENTS, TO ELECTRONS, ING., A' CORPORATION 0F DELAWARE Application led June 6, 1927, Serial No. 196,996, and in Germany June 9, 1926.

Owing to Wehnelts researches it is known that even at a very low temperature the oxides, chlorides and luorides of the alkaline earth metals and mixtures thereof emit electrons. Subsequently, all simply composed compounds of the elements concerned ave been found to be likewise possessed of this characteristic. Among all. these com* pounds only the oxide has become of practical importance. -When` in practice 'other materials were used to commence with such, for-example, as the nitrate (according to the old method of 'Wehnelt), the oxide formed by decomposition of the initial material was always the -emitting substance. Now, those compounds which even at a particularly low temperature have a very high emission, for` example the compounds of barium which is very electropositive and which, in this respect, even closely resembles" the.alkali metals, are possessed of the objectionable characteristic that they are little resistant to the influence of the glow discharge and easily volatilize. Moreover, it should not remain unmentioned that according to our recent researches the volatilization of the oxides depends to a marked extent on the substratum. used.

Even now the manufacture of the Wehnelt cathode is still beset with very considerable difficulties. During the manufacture the oxides change very easily, for example, by reason of the incorporation of carbonio acid. It is therefore, very difficult to ensure with the oxides the necessary uniformity of product in the manufacture and losses occur which increase very much the expense of the process.

The endeavours to use for manufacturing incandescent cathodes instead of the sensitive oxides the much less sensitive and more lasting salts` of the oxyacids of the alkaline earth metalshave failed. The compounds under consideration such as the sulphates, silicates, phosphates and carbonates'have no emission since the highly electronegative acid residue or radical prevents entirely the issue of the electrons. Not until the salts dissociate i. e. when part of the frequently volatile acid residue is isolated at a high temperature and consequently free oxide is present, a perceptible electronic emission ensues. The carbonate, which is most easily dissociable of all these compounds, yields with careful and full dissociation a useful cathode but the process takes much time and is beset with diliiculties.

Now, by an accurate research on these circumstances it was found that yet quite a series of salts of the oxyacids and even salts of non-volatile oxyacids are possessed to a very marked extent of the characteristie of emitting electrons even at a very low temperature. The salts under consideration are the compounds of the oxides of the highly electropositive metals, with those oxygen compounds of the elements which are referred to as anhydrides of amphoteric.

compounds less Vacid than titanic acid, consequently said compounds are, for example, the aluminates, zincates, chromites, zirconiates,l etc. of the highly electropositive metals. By highly electropositive metals, it is meant alkaline and alkaline earth metals. At'increasing acidity of the acid residue the capability of the substance to emit electronsu decreases. Thus, the alkaline earth metal salt of Zirconic acid can still be referred to as an excellent emitting substance whereas the titanate has no useful electronic emission. Oxides whicharev more acid than titanic acid anhydride are`therefore not suitable for the corresponding acid residues, the electron aliinity of the acid residue being in that case already so high that the emission electrons (valence electrons) are too strongly bound. The emission temperature of these compounds is but inimaterially higher than of the alkaline earth metal oxide emitters heretofore know-n. These new compounds have however a stability with respect to volatilization with temperature increase above the normal temperature of effective electron emission that is not possessed by the simple compounds, for example, the oxides. 'In contradistinction to barium which if applied as an oxide to a platinum substratum soon volatilizes under the influence of the glow discharge current, the above compounds are capable,

even after lon use, to withstand substantially heavierl ischarges.

In the drawing, a thermionic tube is shown containing an anode Vand a cathode having a core 1 and a 'coating 2 of a salt as described in the above.

For the manufacture of the cathodes, the compounds herein disclosed may be prepared beforehand (see manual off Gmelin entitled Gmelin-Krauts Handbook for Anorganic Chemistry, published at Heidelberg, Germany, Carl Winter, editor) and applied to the substratum or c'ore. It is however, desirable that the compounds should be prepared by chemical combination taking place on the cathode core. Thus, the advantage is `obtained that substances can be started with which are absolutely resistant to air. For preparing the aluminates or zirconiates of the alkaline metals the course may be adopted to apply to the substratum thc alkaline earth metal carbonates with hydrated or anhydrous aluminium or `.zirconium oxide respectively and a small quantity of an agglutinant and to heat subsequently, since the carbonio acid is easily driven out in a vacuum, the corresponding salt being formed at the same time.

It is, however, also possible to start without difficulty with the oxides or hydroxides of the alkaline earth metals, the carbonio acid incorporated during the manufacture being-easily and rapidly expelled by the action of the acid oxide in the presence of heat.

A further possibility of preparing the compounds consists in making an alloy or a powder mixture of the, two metals, e. g. barium and zirconium and oxidizing this composition. y f,

The slats just described also may be used in conjunction with other electron emitting substances, and it is even possible in order to increase their mechanic strength or their stability in respect to temperature variations, to add supporting substances, for example, alkaline earth metal fluorides or an excess of aluminium oxides.

The favourable influence which we have recently observed of certain core materials on the durability of the oxide cathodes is quite likely to be due to the compound above described being formed to a small extent on the surface of the core.

. .Examples of the resultant salts herein disclosed may be referred to briefly as leach being a salt of an amphoteric acid less acid than titanic acid, and in which salt the metal element outside the acid Aradical is highly 'eletropositive, or is one ofthe alkali and alkali earth metals, or is a metal at least as electropositive as calcium, or is a metal of the category which consists of potassium, rubidium, caesium, calcium, strontium and barium referred to generally in the art as the alkaline metals. The electron emitter of the present invention may consist of one such salt or a plurality thereof intimately associated. With such salt or salts there may also be intimately associated one or more of the heretofore well known simple binary compound emitters, such as oxides, carbides, or sulphides, for example, barium oxide.

In this specification and its claims, the word amphoteric has its usual meaning, referring to those compounds which may act either as acids or bases, and their anhydrides. Thus barium aluminate is the barium salt of an amphoteric substance in kits acid aspect, whose anhydride is an amphoteric oxide (A1203).

Various features of the invention -described but not claimed herein form the subject matter of copending applications, Serial Nos. 350,771 to' 350,775 inclusive, all filed March 28, 1929.

liVhat we claim is:

1. In an electron tube containing a gaseous medium, in combination, an incandescent cathode for discharge devices comprising a core and an emitting substance thereon at least the surface of which is exposed to ionic bombardment, said substance comprising a salt of an alkaline earth metal, said salt having as a component a compound capable of forming by chemical conversion an amphoteric compound whichI is less acid than titanic acid.

2.r In an electron tube, in combination, an incandescent cathode for discharge devices comprising a core and an emitting substance exposed to bombardment thereon having distributed throughout its mass a salt of a highly electro-positive metal, said .salt havingas a component a compound capable of forming by chemical conversion an amphoteric conpound which is less acid than titanic acl 3. An incandescent `cathode according to claim 2 in which a plurality of diderent ones of said salts are mixed to form an emitting substance.

4 In an electron tube containing a gaseous medium, in combination, a cathode comprising a core and an incandescent electron emissive substance thereon exposed to ionic bombardment and comprising, at least at the surface of said substance the compound of a highly electro-positive metal oxide and a metal oxide which is capable of forming by chemical conversion an amphoteric compound less acid than titanic acid. l

5. An electric discharge tube containing a gaseous medium having an incandescent electron emissive cathode, said cathode consisting of a core and a compound exposed to ionic bombardment, the element of which compound is a highly electro-positive metal tron emissive cathode, said cathode consisting of a core and a compound exposed to ionic bombardment, the element of which compound is a highly electro-positive metal and the radical of which consists of a mixture of oxide of aluminum and oxide of zirconium.

7. In an evacuated vessel, in combination, an anode and a cathode, said cathode being subjected to destructive forces and consisting of a core and an electron emissive and toughened material thereon exposed to ionic bombardment and including an emissively effective salt of a compound from which an amphoteric compound less acid than titanic acid may be chemically produced in association with aluminum oxide, said aluminum' oxide being present in excess of the amount entering into chemical combination with the other ingredients of the said material.

8. In an electron emitting tube, in combination, an anode and a cathode, said cathode` consisting of a core and an electron emissive material thereon exposed to ionic bombardment including an emissively effective salt of a compound capable of forming by chemical conversion an edective electron emissive salt of an amphoteric compound less acid than titanic acid in association with an alkaline earth metal fluoride and aluminum oxide, said aluminum oxide being present in excess of the amount entering into chemical combination in the material.

9. In an electron tube, in combination, an anode and a cathode, said cathode consisting of an electron emissive cathode, operable at moderate'temperature and rugged to endure positive of a core and a coating of barium aluminate.

10. An incandescent cathode according to claim 2 containing a supporting substance such as an alkaline earth metal iiuoride.

11. In an electron tube, in combination, an incandescent cathode for discharge devices comprising a core and an emitting substance exposed to lbombardment thereon having at least on its surface a salt of a highly electropositive metal, said salt having as a component a compound capable of forming by chemical conversion an amphoteric compound which is less acid than titanic acid, said coating also comprising an ingredient to give mechanical strength and resistance to temperature variations.

12. The process of making an electron emissive cathode operable at moderate temerature and rugged to endure positive ion mbardment which consists in preparing a metal core, then coating said core with a composition at least the outer portion of ion bombardment, consisting which comprises barium carbonate, aluminum oxide, an alkali earth metal fluoride, an alkali earth metal oxide and an agglutinant, then heating the coating core in the absence of oxygen, whereby carbon dioxide is liberated from said carbonate and its residue unites with the aluminum oxide to form an essential electron emissive component of the coating, said coating being strengthened mechanically and made more resistant to temperature variations by the presenceof said fluoride.

13. In an' electron tube, in combination,

van anode and a cathode, said cathode consisting of an electron emissive substance operable at moderate temperatures and rugged to endure positive ion bombardment to which it is subjected consisting of a core and a coating comprising strontium aluminate.

14. An electricvdischarge tube containing a gaseous medium having an incandescent cathode consisting of a core and coating exposed to ionic bombardment, said coating comprising a compound of barium oxide, said compound having as a component a compound capable of forming by chemical conversion an amphoteric compound less acid than titanic acid and a compound of strontium oxide, said compound having a a compound component a compound capable of forming by chemical conversion an amphrteric compound less acid than titanic aci In testimony whereof we aiix our signatures, at the city of Berlin, this 20th of May,-

FRIEDRICH MEYER. HANS-JOACHIM SPANNER. 

